Test circuit having parallel drive segments and a plurality of sense elements

ABSTRACT

A test circuit having a drive winding with parallel conducting segments and a plurality of sense elements used for the nondestructive measurement of materials. The drive winding segments have extended portions and are driven by a time varying electric current to impose a magnetic field in the test material. Sense elements are distributed in a direction parallel to the extended portions of these drive segments, with separate connections provided to each sense element. A second plurality of sense elements may also be distributed parallel to the extended portions of the drive windings, being either aligned or offset from a first plurality of sense elements.

RELATED APPLICATION

This application is a divisional of U.S. application Ser. No. 10/102,620filed Mar. 19, 2002, now U.S. Pat. No. 6,784,662 which claims thebenefit of U.S. Provisional Application No. 60/276,997 filed Mar. 19,2001, the entire teachings of which are incorporated herein byreference.

BACKGROUND

The technical field of this invention is that of nondestructivematerials characterization, particularly quantitative, model-basedcharacterization of surface, near-surface, and bulk material conditionfor flat and curved parts or components using eddy-current sensors.Characterization of bulk material condition includes (1) measurement ofchanges in material state caused by fatigue damage, creep damage,thermal exposure, or plastic deformation; (2) assessment of residualstresses and applied loads; and (3) assessment of processing-relatedconditions, for example from shot peening, roll burnishing,thermal-spray coating, or heat treatment. It also includes measurementscharacterizing material, such as alloy type, and material states, suchas porosity and temperature. Characterization of surface andnear-surface conditions includes measurements of surface roughness,displacement or changes in relative position, coating thickness,temperature and coating condition. Each of these also includes detectionof electromagnetic property changes associated with single or multiplecracks. Spatially periodic field eddy-current sensors have been used tomeasure foil thickness, characterize coatings, and measure porosity, aswell as to measure property profiles as a function of depth into a part,as disclosed in U.S. Pat. Nos. 5,015,951 and 5,453,689.

Conventional eddy-current sensing involves the excitation of aconducting winding, the primary, with an electric current source ofprescribed frequency. This produces a time-varying magnetic field at thesame frequency, which in turn is detected with a sensing winding, thesecondary. The spatial distribution of the magnetic field and the fieldmeasured by the secondary is influenced by the proximity and physicalproperties (electrical conductivity and magnetic permeability) of nearbymaterials. When the sensor is intentionally placed in close proximity toa test material, the physical properties of the material can be deducedfrom measurements of the impedance between the primary and secondarywindings. Traditionally, scanning of eddy-current sensors across thematerial surface is then used to detect flaws, such as cracks.

In many inspection applications, large surface areas of a material needto be tested. This inspection can be accomplished with a single sensorand a two-dimensional scanner over the material surface. However, use ofa single sensor has disadvantages in that the scanning can take anexcessively long time and care must be taken when registering themeasured values together to form a map or image of the properties. Theseshortcomings can be overcome by using an array of sensors or an array ofelements within a single sensor, as described for example in U.S. Pat.No. 5,793,206, since the material can be scanned in a shorter period oftime and the measured responses from each array element are spatiallycorrelated. However, the use of arrays complicates the instrumentationused to determine the response of each array element. For example, inone conventional method, as described for example in U.S. Pat. No.5,182,513, the response from each element of an array is processedsequentially by using a multiplexer for each element of the array. Whilethis is generally faster than scanning a single sensor element, there isstill a significant time delay as the electrical signal settles for eachelement and there is the potential for signal contamination frompreviously measured channels.

For nondestructive testing of conducting and/or magnetic materials overwide areas, eddy current sensor arrays may be used. These eddy currentsensors excite a conducting winding, the primary, with an electricalcurrent source of a prescribed frequency. This produces a time-varyingmagnetic field at the same frequency, which in turn is detected with asensing winding, the secondary. The spatial distribution of the magneticfield and the field measured by the secondary is influenced by theproximity and physical properties (electrical conductivity and magneticpermeability) of nearby materials. When the sensor is intentionallyplaced in close proximity to a test material, the physical properties ofthe material can be deduced from measurements of the impedance betweenthe primary and secondary windings. Traditionally, scanning ofeddy-current sensors across the material surface is then used to detectflaws, such as cracks. When scanning over wide areas, these arrays mayinclude several individual sensors, but each sensor must be drivensequentially in order to prevent cross-talk or cross-contaminationbetween the sensing elements.

Eddy current arrays have also been disclosed in U.S. Pat. No. 5,262,722,however the implemented versions of these arrays use differentialsensing elements. The use of differential sensing element, thatessentially compare the response of two neighboring sensing regions,limits the capability to determine absolute properties of interest.These sensor arrays and conventional eddy current sensors are alsohighly sensitive to sensor position, requiring expensive automatedscanners to build images of material properties for complex surfaceinspections. Differential sensors may also produce false indications onrelatively rough surfaces, such as surfaces with fretting damage.

SUMMARY

Aspects of the inventions described herein involve novel sensors for themeasurement of the near surface properties of conducting and/or magneticmaterials. These sensors use novel geometries for the primary windingand sensing elements that promote accurate modeling of the response andprovide enhanced capabilities for the creation of images of theproperties of a test material.

In one embodiment, sensor array designs are disclosed that permit thecreation of property images when scanned over a material surface. In oneembodiment, the drive winding includes at least one central conductingsegment and parallel return segments located on either side to impose aperiodic magnetic field of at least two spatial wavelengths in a testmaterial, a linear array of sensing elements to sense the response tothe test material properties, and at least one sensing element uses amagnetoresistive (MR) or giant magnetoresistive (GMR) sensor. Secondarycoils can also be placed around one or more of the MR or GMR senseelements, in one embodiment. In another, these coils are connected in afeedback configuration, and, in one embodiment, act to maintain themagnetic field at the MR or GMR sensor at a prescribed level.

In another embodiment of a sensor array design, the drive windingincludes at least one central conducting segment and at least oneparallel return segment on either side, a linear array of sensingelements between the central segments and a return segment, and separateconnections to each sensing element. The distance between the centralsegments and the return segments can be selected to align with featuresof interest in a test material, such as bolt holes. One embodimentincludes two central conductors and a return path for each conductor,with equal distances between the central conductors and each returnpath. In another embodiment, a second linear array of sensing elementsis placed between another pair of linear drive winding segments,parallel to the first linear array. In one form, each element in thefirst array is aligned with an element in the second array. In anotherform, elements in the first array are offset from the elements in thesecond array in a direction parallel to the linear drive windingsegments. Preferably, this offset distance is one-half of the length ofa sense element, which ensures complete coverage of the element in adirection perpendicular to the drive winding segments. In an embodiment,the linear arrays are equally distant from the central conductors.Differential measurements may also be taken in the response betweenelements in the first array and elements in the second array. Thecentral conductors can be placed in the same plane as the sensingelements to improve the coupling with the sense elements.

In an embodiment, the conductivity and proximity of the sensing elementsto the surface are measured to detect cracks. In another, the proximityof each sensing element to the test material surface is used todetermine surface roughness. In another embodiment, the sensing elementresponse is used for health monitoring or condition assessment of acomponent. An embodiment also includes the use of a characteristicsensor response for a flaw and using that characteristic response toconstruct a filter. This filter can be applied to a response image toemphasize indications that are likely to be associated with flaws andsuppresses indications unlikely to be associated with flaws.

In one embodiment, a single encoder determines the position of the arraywhile scanning. In another embodiment, an automated scanner is used tomove the array over a test material. In another embodiment, usingmodular fixtures with position encoders facilitates manual scanning ofcomplex parts. In an embodiment, a template is used to align incrementalscans over a test material so that images of the material propertiesover areas wider than the array width can be generated.

To facilitate the scanning of a sensor array over a material testsurface, another embodiment includes the use of a fluid filled balloon.In an embodiment, this balloon is attached to a shuttle and the shuttleis shaped to approximately match the shape of the test material. Inanother embodiment, the sensor and balloon components are modularizedand can be replaced rapidly. In one embodiment, the inspection isperformed on the surface of a bolt hole. In another, the inspection isperformed on the inside of an engine disk slot.

In another embodiment of a sensor array design, the drive windingincludes at least one pair of parallel conducting segments to impose amagnetic field in the test material, a linear array of sensing elements,and separate connections to each sensing element. The distance betweenthe parallel segments can be selected to align with features of interestin a test material. In one form, the linear array is placed between theparallel segments of the drive. Preferably, in another form, the arrayis placed outside the loop formed by the parallel segments of the drive.This also permits both the drive segments and the sense elements to beplaced in the same plane.

In another embodiment, a second linear array of sensing elements isplaced parallel to the first linear array of elements. This second arraycan be placed between the parallel segments of the drive winding, near asegment of the drive winding common with the first array. In anotherembodiment, the second array is placed outside the drive winding loop,opposite that of the first array. The distances between the lineararrays and the drive winding segments can be selected to be the same ordifferent. In one embodiment, each element in the first array is alignedwith an element in the second array. In another embodiment, elements inthe first array are offset from the elements in the second array in adirection parallel to the linear drive winding segments. Preferably,this offset distance is one-half of the length of a sense element.

In an embodiment for the sensor array, the locations of the sensingelements in a direction parallel to the drive segments and the sensingelement size can be made non-uniform to provide a higher imageresolution over specific material test areas. In another embodiment, thesensor array can be fabricated onto a flexible substrate so that thesensor can conform to the shape of the test material. Alternatively, thesensor array can be fabricated onto a rigid substrate. With eithersubstrate material, measurements of the material properties can beperformed in a noncontact fashion. In an embodiment, at least one of thesensing elements includes a MR or GMR sensor. In one form, these sensingelements also include a secondary coil. In another form, the secondarycoil is used in a feedback configuration.

In yet another embodiment, a sensor array design comprises two parallellinear rows of sensing elements on opposite sides of a central conductorfor detecting cracks on each side of a feature. In one embodiment thisfeature is a fastener in an aircraft skin. In another embodiment,multiple frequency measurements are used to remove interference cause bythe feature itself to isolate and emphasize the response of the crack.An embodiment also includes using the sensor response from a sensingelement to create a characteristic response for a flaw and to constructa filter. This filter can be applied to a response image to emphasizeindications that are likely to be associated with flaws and suppressesindications unlikely to be associated with flaws. In one form, the flawis a crack. In another, the flaw is a buried inclusion.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

FIG. 1 is a drawing of a spatially periodic eddy current sensor array.

FIG. 2 is an expanded view of the drive and sense elements for thespatially periodic eddy current sensor array shown in FIG. 1.

FIG. 3 is a pictorial cross-sectional view of some of the drive andsense elements for the sensor of FIG. 1.

FIG. 4 is a plot of the calculated sensor response to a notch as the gapbetween the sensing elements and the central primary conductors isvaried.

FIG. 5 is an unfiltered measurement image taken with an eddy currentsensing array over a Titanium alloy plate containing cracks at afrequency of 8 MHz.

FIG. 6 is an unfiltered measurement image taken with an eddy currentsensing array over a Titanium alloy plate containing cracks at afrequency of 12 MHz.

FIG. 7 is a filtered measurement image that combines the data of FIG. 5and FIG. 6 to highlight the cracks.

FIG. 8 is a plot of the unfiltered 8 MHz sensor response from element 7in the trailing row of elements in the array used to scan over aTitanium alloy plate containing cracks.

FIG. 9 is a plot of the 8 MHz sensor response to a single crack in aTitanium plate.

FIG. 10 is a plot of the filtered sensor response from element 7 usingthe shape responses like those of FIG. 9 and both measurementfrequencies.

FIG. 11 is a drawing of a spatially periodic field eddy current sensorarray having all connection leads on one side of the array.

FIG. 12 is a drawing of an eddy current sensing array being near anopening in a test material.

FIG. 13 is a drawing of an eddy current sensing array being insertedinto a pipe.

FIG. 14 is a cross-sectional view of an eddy current sensing arrayinside a pipe.

FIG. 15 is another drawing of an eddy current sensing array being nearan opening in a test material.

FIG. 16 is another drawing of an eddy current sensing array beinginserted into a pipe.

FIG. 17 is a drawing of a single wavelength eddy current sensor array.

FIG. 18 is an expanded view of the drive and sense elements for the eddycurrent array shown in FIG. 17.

FIG. 19 is a pictorial cross-sectional view of the drive and some of thesense elements for the eddy current array shown in FIG. 17.

FIG. 20 is an expanded view of the drive and sense elements for an eddycurrent array having offset rows of sensing elements.

FIG. 21 is a plot of the calculated response to a surface breaking notchusing a model, indicating the response to the secondary element on theleft side of the central conductor.

FIG. 22 is an expanded view of the drive and sense elements for an eddycurrent array having a single row of sensing elements.

FIG. 23 is a schematic of the normalized conductivity for a measurementchannel of a high-resolution MWM-Array with longer segments of theprimary winding oriented parallel to the weld axis for a similar metalzero LOP defect specimen.

FIG. 24 is an expanded view of an eddy current array where the locationsof the sensing elements along the array are staggered.

FIG. 25 is an expanded view of an eddy current array with a singlerectangular loop drive winding and a linear row of sense elements on theoutside of the extended portion of the loop.

FIG. 26 is a schematic for an eddy current array with a singlerectangular loop drive winding and two rows of sense elements on theoutside of the extended portions.

FIG. 27 is a schematic for an eddy current array with differentdistances between each row of sensing elements and the drive winding.

FIG. 28 is a schematic for an eddy current array with a spatial offsetbetween each row of sensing elements, parallel to the extended portionsof the drive winding.

FIG. 29 is a schematic for an eddy current array having a row of sensingelements inside the drive winding loop and a row of sensing elementsoutside the drive winding loop.

FIG. 30 is a schematic for an eddy current array with an electroniccircuit at one end of the primary winding loop.

FIG. 31 shows a conductivity image for an aluminum bending fatiguespecimen obtained from an MWM-Array scanned with the array driveperpendicular to the specimen axis.

FIG. 32 shows a conductivity image for an aluminum bending fatiguespecimen obtained from an MWM-Array scanned with the array driveparallel to the specimen axis.

FIG. 33 shows the conductivity/lift-off measurement grid used to producedata in FIG. 32.

FIG. 34 shows a plate thickness image for a floor chine plate obtainedwith an MWM-Array and a thickness/lift-off measurement grid.

FIG. 35 shows another plate thickness image for a floor chine plateobtained with an MWM-Array and a thickness/lift-off measurement grid.

FIG. 36 shows another thickness image of the same data as in FIG. 35with the image scale highlighting low to intermediate corrosion lossregions.

FIG. 37 shows MWM-Array generated images of the 5 percent maximummaterial loss, represented by a dome-shaped cavity on the inside firstlayer surface (left image) between two 0.04-in. thick aluminum skins;inside second layer surface (right image) between two 0.04-in. thickaluminum skins.

FIG. 38 shows a measurement grid and responses of a single channel of anMWM-Array to material loss between two layers as the sense element isscanned across the loss region for first layer thinning.

FIG. 39 shows a measurement grid and responses of a single channel of anMWM-Array to material loss between two layers as the sense element isscanned across the loss region for second layer thinning.

FIG. 40 shows a plot of first and second layer material loss forindividual MWM sensing elements scanned across the maximum loss pointfor reported 5 percent and 10 percent material loss.

FIG. 41 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 1 MHzand the extended portions of the primary winding oriented parallel tothe loading axis.

FIG. 42 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 158 kHzand the extended portions of the primary winding oriented parallel tothe loading axis.

FIG. 43 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 1 MHzand the extended portions of the primary winding oriented perpendicularto the loading axis.

FIG. 44 shows a normalized permeability image obtained from an MWM-Arrayscanned over a double-notched 4340 low-alloy steel tensile specimenafter failure in a tension test, with an excitation frequency of 158 kHzand the extended portions of the primary winding oriented perpendicularto the loading axis.

FIG. 45 shows a permeability/lift-off measurement grid and data from asingle element of an MWM-Array.

FIG. 46 shows a schematic of MWM-Rosette designed for detection ofcracks at fasteners.

FIG. 47 shows a linear MWM-Array used to monitor crack initiation andgrowth along a linear feature.

FIG. 48 shows a linear MWM-Array used to detect cracks that propagateacross a specific location within a structural member.

FIG. 49 shows data from a fatigue test with an MWM-Rosette mountedaround a hole in an aluminum dogbone specimen, the test being stoppedshortly after the crack reached channel 6.

FIG. 50 shows the crack size vs. number of load cycles based on the testdata shown in FIG. 49.

FIG. 51 shows the structure of a rotationally symmetric shaped fielddrive winding.

FIG. 52 shows results of conductivity/lift-off measurements with thecircular magnetometer.

FIG. 53 shows an area scan of a stainless steel plate with the crack atthe surface.

FIG. 54 shows the structure of the hybrid sensor feedback loop.

FIG. 55 shows multiple GMR sensors placed within a feedback coil and atthe center of a drive winding.

FIG. 56 shows multiple GMR sensors placed within a feedback coil andoffset near an edge of a drive winding.

FIG. 57 shows two linear arrays of GMR sensors placed within feedbackcoils and external to the drive winding.

DETAILED DESCRIPTION OF THE INVENTION

A description of preferred embodiments of the invention follows. Thedesign and use of high resolution conformable eddy current sensor arraysis described for the nondestructive characterization of materials. Thesesensor arrays are well suited to inspections over wide areas as a singlescan of the sensor array allows the material properties to be determinedover a relatively wide distance. Also, sequential scans can beconcatenated, with or without overlap, to create images over wide areas.Furthermore, simple manual scans can be used with only a roller encoderto record position, still producing two-dimensional images of thequality previously achieved with high cost automated scanners.Measurements of the responses from each element in a linear array ofsensing elements, oriented perpendicular to the scan direction, alsofacilitates the creation of material property images so that thepresence of property variations or defects are readily apparent.

In one embodiment, eddy current sensor arrays with at least onemeandering drive winding and multiple sensing elements are used toinspect the test material. An example sensor array is shown in FIG. 1.Expanded views of the region near the sensing elements are shown in FIG.2 and FIG. 3. This array includes a spatially periodic primary winding70 having extended portions for creating the magnetic field and aplurality of secondary elements 72 within the primary winding forsensing the response to the material under test (MUT). The primarywinding is fabricated in a periodic pattern with the dimension of thespatial periodicity termed the spatial wavelength λ. This geometry canbe described as a meandering winding so that a single element sensor,where all of the sensing elements are connected together, can be calleda Meandering Winding Magnetometer (MWM®) and a sensor array having asimilar primary winding an MWM-Array, as described in U.S. patentapplication Ser. No. 10/010,062, filed Nov. 13, 2001, the entireteachings of which are incorporated herein by reference. Melcher firstconceived the use of meandering or rectangular drives with multiplesensing regions and drive wires connected in series to cover asignificant area, as described in U.S. Pat. No. 5,015,951. Detaileddescriptions of this geometry for an eddy current sensor are given inU.S. Pat. Nos. 5,453,689, 5,793,206, and 6,188,218. In U.S. Pat. No.5,262,722, a similar approach to Melcher's is used to link seriesconnected drive regions to excite differential sensing elements. In theMWM sensors, a time-varying current is applied to the primary winding,which creates a magnetic field that penetrates into the MUT and inducesa voltage at the terminals of the secondary elements. This terminalvoltage reflects the properties of the MUT. The secondary elements arepulled back from the connecting portions of the primary winding tominimize end effect coupling of the magnetic field. Dummy elements 74can be placed between the meanders of the primary to maintain thesymmetry of the magnetic field, as described in U.S. Pat. No. 6,188,218.The magnetic vector potential produced by the current in the primary canbe accurately modeled as a Fourier series summation of spatialsinusoids, with the dominant mode having the spatial wavelength λ. Foran MWM-Array, the responses from individual or combinations of thesecondary windings can be used to provide a plurality of sense signalsfor a single primary winding construct as described in U.S. Pat. Nos.5,793,206 and Re. 36,986 and also U.S. application Ser. No. 09/666,879,filed Sep. 20, 2000, now U.S. Pat. No. 6,657,429, the entire teachingsof which are incorporated herein by reference, and U.S. application Ser.No. 09/666,524, filed Sep. 20, 2000, the entire teachings of which areincorporated herein by reference.

The MWM structure can be produced using micro-fabrication techniquestypically employed in integrated circuit and flexible circuitmanufacture. This results in highly reliable and highly repeatable(i.e., essentially identical) sensors, which has inherent advantagesover the coils used in conventional eddy-current sensors which exhibitsignificant sensor-to-sensor variability even for nominally identicalsensors. As indicated by Auld and Moulder, for conventional eddy-currentsensors “nominally identical probes have been found to give signals thatdiffer by as much as 35%, even though the probe inductances wereidentical to better than 2%” (Auld, 1999). The lack of reproducibilitywith conventional coils introduces severe requirements for calibrationof the sensors (e.g., matched sensor/calibration block sets). Incontrast, duplicate MWM sensor tips have nearly identical magnetic fielddistributions around the windings as standard micro-fabrication(etching) techniques have both high spatial reproducibility andresolution. As the sensor was also designed to produce a spatiallyperiodic magnetic field in the MUT, the sensor response can beaccurately modeled which dramatically reduces calibration requirements.For example, in some situations an “air calibration” can be used tomeasure an absolute electrical conductivity without calibrationstandards, which makes the MWM sensor geometry well-suited to surfacemounted or embedded applications where calibration requirements will benecessarily relaxed.

An efficient method for converting the response of the MWM sensor intomaterial or geometric properties is to use grid measurement methods.These methods map the magnitude and phase of the sensor impedance intothe properties to be determined and provide for a real-time measurementcapability. The measurement grids are two-dimensional databases that canbe visualized as “grids” that relate two measured parameters to twounknowns, such as the conductivity and lift-off (where lift-off isdefined as the proximity of the MUT to the plane of the MWM windings).For the characterization of coatings or surface layer properties,three-dimensional versions of the measurement grids can be used.Alternatively, the surface layer parameters can be determined fromnumerical algorithms that minimize the least-squares error between themeasurements and the predicted responses from the sensor.

An advantage of the measurement grid method is that it allows forreal-time measurements of the absolute electrical properties of thematerial. The database of the sensor responses can be generated prior tothe data acquisition on the part itself, so that only table lookupoperation, which is relatively fast, needs to be performed. Furthermore,grids can be generated for the individual elements in an array so thateach individual element can be lift-off compensated to provide absoluteproperty measurements, such as the electrical conductivity. This againreduces the need for extensive calibration standards. In contrast,conventional eddy-current methods that use empirical correlation tablesthat relate the amplitude and phase of a lift-off compensated signal toparameters or properties of interest, such as crack size or hardness,require extensive calibrations and instrument preparation.

While a single meandering conductor can be used for the primary winding,this leads to the formation of a large inductive loop that can influencethe eddy current sensor response. Splitting the primary winding so thatthe return leads to each component of the drive winding are in closeproximity to one another can substantially reduce the effects of thisextraneous inductive loop. In FIG. 1, FIG. 2 and FIG. 3, the primarywinding 70 is split into two parts so that each extended portion of aprimary winding meander, except for the endmost, includes two conductingelements. Each loop of the primary winding is connected together inseries and the primary windings are wound so that the current throughadjacent conductors is in the same direction. The current through thesetwo conducting loops imposes a spatially periodic magnetic field. Thiswinding configuration minimizes the effects of stray magnetic fieldsfrom the lead connections to the primary winding, which can create anextraneous large inductive loop that influences the measurements,maintains the meandering winding pattern for the primary, andeffectively doubles the current through the extended portions of themeanders. This method for reducing the effects of the extraneous loop isdescribed more completely in U.S. application Ser. No. 09/666,879, nowU.S. Pat. No. 6,657,429, and Ser. No. 09/666,524.

When the sensor is scanned across a part or when a crack propagatesacross the sensor, perpendicular to the extended portions of the primarywinding, secondary elements 76 in a primary winding loop adjacent to thefirst array of sense elements 72 provide a complementary measurement ofthe part properties. These arrays of secondary elements 76 are alignedwith the first array of elements 72 so that images of the materialproperties will be duplicated by the second array. Alternatively, toprovide complete coverage when the sensor is scanned across a part thesensing elements 76 can be offset along the length of the primary loopor when a crack propagates across the sensor, perpendicular to theextended portions of the primary winding, secondary elements 76 in aprimary winding loop adjacent to the first array of sense elements 72can be offset along the length of the primary loop, as illustrated inFIG. 20. Additional primary winding meander loops, which only containdummy elements, are placed at the edges of the sensor to help maintainthe periodicity of the magnetic field. The connection leads 78 to thesecondary elements are perpendicular to the extended portions of theprimary winding, which necessitates the use of a multi-layer structurein fabricating the sensor. The layers that contain the primary andsecondary winding conductors are separated by a layer of insulation.Layers of insulation are generally also applied to the top and bottomsurfaces of the sensor to electrically insulate the primary andsecondary windings from the MUT. A protective layer is also sometimesused, e.g. Kapton™ or Teflon™ with or without a removable adhesive. Thislayer becomes sacrificial, protecting the sensor and being periodicallyremoved and replaced with age.

The leads to the primary and secondary elements are kept close togetherto minimize fringing field coupling. The leads 82 for the primarywinding are kept close together to minimize the creation of fringingfields. The leads 78 for the secondary elements are kept close togetherto minimize the linkage of stray magnetic flux. The bond pads 86 providethe capability for connecting the sensor to a mounting fixture. The bondpads 86 are spread out for easier design, contact, and assembly of theconnectors to the bond pads. The trace widths for the primary windingcan also be increased to minimize ohmic heating, particularly for largepenetration depths that require low frequency and high current amplitudeexcitations. Also, the conducting primary thickness and width may beincreased to minimize ohmic heating.

The placement of the sensing elements near the primary windings can alsobe adjusted to enhance sensitivity to specific types of flaws ordefects. FIG. 2 shows an expanded view around the sensing elements forthe sensing array of FIG. 1. The arrays of sensing elements 72 and 76are located relatively close to the common portions of the primarywinding 70 so that the distance 80 is smaller than the distance 81. PastMWM designs emphasized placing the sensing elements at the center of thegap between the primary winding legs, so that the distances 80 and 81were equal, to minimize coupling of short spatial wavelength magneticfield modes. With the previous design, scanning the sensor array over asmall (compared to a half-wavelength) surface breaking or near-surfacedefect leads to a double-humped response from the sensing element, witheach hump occurring when the drive windings nearest to the sensingelements are predominantly over the defect. Physically, for a conductingMUT, the time varying magnetic field induces eddy currents in the MUTthat mirror the conductor pattern of the primary winding and theseinduced eddy currents are largest beneath the primary windings. Thepresence of a defect interrupts this current flow and the perturbationsin the magnetic field are detected with the sensing elements. With thenew offset secondary design (distances 80 and 81 unequal), the responseto the defect will be asymmetric with an enhanced response when thedefect is beneath the common or central primary winding 70 and a reducedresponse when the defect is beneath the further or return primarywindings. FIG. 4 illustrates this effect. Modeling was performed tocalculate the response in the relative phase change as a flaw, in thiscase a rectangular notch, passes beneath a single element of an array.Increasing the gap between the central conductor and the return windingcauses a decrease in the response peaks for flaw locations beneath thecentral primary conductor and the secondaries but an increase in theresponse peak for flaw locations beneath the return winding. Theresponse when the defect is beneath the return portion of the primarywinding can be reduced by moving the return winding further away fromthe sensing elements, as shown in FIG. 21. Reducing the response fromthe return is an advantage when trying to build images and improvereliability.

The arrays of sensing elements 72 and 76 are offset the same distance 80from the common primary winding 71 to maintain the capability forobtaining the same measurement from a given defect. Material propertyvariations and orientation of non-spherical defects can affect theresponses of the sensing elements in each array differently, whichprovides the potential to separate defect features and defect signalsfrom background property variations. A simple filter would be to sum theresponses of spatially correlated sensing elements (when the scanningdirection is perpendicular to the extended portions of the primarywinding), which would highlight the presence of defects when underneaththe common drive winding 71. Furthermore, filters can compare thesensing element responses to ensure that the spatially correlatedsensing elements are responding to the same feature. In addition, theresponses of neighboring sensing elements can be used to normalize theresponse, eliminating background variations of the material properties.Multiple frequency measurements can also be used to enhance the results.

The distinct shapes of the sensor response when passing over a flaw canbe isolated using “matched filters” as described in application Ser. No.10/010,062, now abandoned. Then, by searching an image for thisdistinctive shape the response to a local defect can be enhanced. As anexample, this process is illustrated in FIG. 5 through FIG. 10 forcracks in a Titanium alloy flat crack standard. Unfiltered images of theeffective conductivity images from a scan over the standard are shown inFIG. 5 for an 8 MHz excitation and in FIG. 6 for a 12 MHz excitation. Onthis particular standard, there are three cracks of lengths 0.711,0.635, and 0.686 mm (0.028, 0.025, and 0.027-inches, respectively) alongthe path of element 7. A filtered image, shown in FIG. 7, hashighlighted cracks and suppressed background noise variations.

In this case the filtered image combines the response from both thetrailing and leading rows of sensing elements at both measurementfrequencies into a single response. This is accomplished for each row(trailing and leading) and measurement frequency (8 MHz and 12 MHz) byfirst calculating, element-by-element, the correlator of a moving windowof data with a shape filter. For example FIG. 8 shows the unfiltered 8MHz data from element 7 in the trailing row and FIG. 9 shows thetrailing row shape response. Similar responses are used for the leadingrow data and the 12 MHz data. The resulting signal can be denotedx₁(i,j) where the index i denotes element number and j denotes themeasurement number. Repeating this process yields x₂(i,j) for the 8 MHzleading row data, y₁(i,j) for the 12 MHz trailing row data, and y₂(i,j)for the 12 MHz leading row data. The results from each row of the 8 MHzdata are then combined as

${x\left( {i,j} \right)} = \frac{\left( {{x_{1}\left( {i,j} \right)} + {x_{2}\left( {i,j} \right)}} \right)}{2*2^{({{x_{1}{({i,j})}} - {x_{2}{({i,j})}}})}}$and the results from each row of the 12 MHz data are then combined as

${y\left( {i,j} \right)} = \frac{\left( {{y_{1}\left( {i,j} \right)} + {y_{2}\left( {i,j} \right)}} \right)}{2*2^{({{y_{1}{({i,j})}} - {y_{2}{({i,j})}}})}}$Then, the results from each frequency are combined as

${z\left( {i,j} \right)} = \frac{\left( {{x\left( {i,j} \right)} + {y\left( {i,j} \right)}} \right)}{2*2^{({{x{({i,j})}} - {y{({i,j})}}})}}$

This result is shown in FIG. 10 for element 7 alone and in FIG. 7 forall of the elements. Note that this particular procedure suppressessignals on one row of elements but not the other row, at a givenfrequency. It also suppresses signals that appear on only one frequencybut not the other. This improves clutter suppression to limit falsealarms.

FIG. 2 and FIG. 3 also show that the connection leads 83 to each sensingelement are closely paralleled by another set of leads 85 ending in aclosed loop 87. As described in U.S. application Ser. No. 09/666,879,now U.S. Pat. No. 6,657,429, and Ser. No. 09/666,524, the differentialresponse between the actual sensing element and the parallel leads 85 ismeasured. This “flux cancellation” configuration provides a measure ofthe absolute signal in the vicinity of the sensing element and helps tominimize the effects of stray inductive and capacitive coupling to thesensing element leads. The use of flux cancellation allows longer leadlines to be used, permits the spreading out of connection leads 83 sothat standard pins can be used for the connections and eliminatescross-talk problems encountered in closely packed connection schemes,and also allows the sensor part of the probe to incorporate a connectionboard. The elimination of tightly packed connectors is a significantcost and durability advantage. Furthermore, this use of a differentialmeasurement to obtain absolute signal responses from the sensingelements permits calibration in air, where calibration coefficients areobtained from comparisons of the sensor signal in air to the predictedresponse for the sensor based on a model for the sensor geometry. Thisthen permits an absolute measurement of the electromagnetic andgeometric properties of the MUT, such as electrical conductivity,magnetic permeability and layer thickness) without the use ofcalibration standards. Of course, the sensor or sensor arrays can alsobe calibrated on reference standard having known properties. Incontrast, the use of conventional differential and absolute eddy currentsensors requires performing calibration measurements on referencestandards to set the gain levels for this instrumentation beforequantitative MUT property information can be obtained. In this designthe primary windings 70 are separated from the secondary element arrays72 and 76 by a layer of insulation 95. This layer of insulation istypically 0.5 to 1 mil (12.7 to 25.4 micrometers) thick Kapton™.

FIG. 11 shows another configuration for an MWM-Array. In this case allof the leads 78 to the sensing element arrays and the leads 82 to theprimary winding are on one side of the sensor. This allows the activearea of the sensor, defined by the area covered by the primary windings70 to be inserted into confined areas such as bolt holes or disk slots.Scanning of the array in a direction perpendicular to the extendedportions of the primary winding, which could require rotating the sensorin a bolt hole, allows complete coverage of the inspection area. Thesensor might also be oriented with the extended portions of the primarywindings at a right angle to that shown or as shown with the sensingregion offset relative to the connector to permit insertion intogeometric features such as the inside surface of pipes, bolt holes, andgun barrels, which is illustrated in FIG. 12 and FIG. 14.

In FIG. 12, the array 132 has a sensing array 130 offset from theconnector (with numerous bond pads) parallel to the direction of thelongest dimension of the connector. The extended portions of the primarywinding for creating the magnetic field are parallel to the offsetdirection of the array. Two arrays of sensing elements are placed at thecenter of and run parallel to the extended portions of the primarywinding. The sensing array is fabricated onto a flexible Kapton™ liningor substrate 136, which permits the shape of the sensing structure to bedeformed for insertion of the eddy current sensing array into confinedareas 134 such as pipes and bolt holes to inspect for defects anddamage. The conformability of the sensing array inside the confinedspace is illustrated in FIG. 13. FIG. 14 shows a cross-sectional view ofthe sensor inside this space. The sensor 132 can be held against theinside surface 142 of the test material 134 with a foam or balloonsupport 140. This support provides both a reasonably rigid framework forholding the sensing structure in-place when inserted into the hole andalso a compliant backing for maintaining intimate contact between thesensing structure and the inside surface of the test material. Thesensor array 130, and ballon support 140 may be rapidly replaced with aremovable cartidge 133. Rotating the sensing structure in acircumferential direction, perpendicular to the extended portions of theprimary winding, then permits complete coverage of the surface of thetest material during the inspection. The inspection can also beperformed with the sensing structure placed at different distances intothe pipe so that pipe lengths greater than the length spanned by thesensing element arrays can be inspected. This also allows multiplemeasurements of a given area to be performed. The distance into the pipecan be monitored with a position encoder or controlled with a roboticarm to permit accurate measurements of the insertion distance into thepipe.

FIG. 15 shows another embodiment for the inspection of confined areassuch as a pipe. In this case the extended portions of the primarywinding, and the arrays of sensing elements 138, are orientedperpendicular to the offset direction of the sensing structure from theconnector. Again, the sensing structure is mounted onto a foam orballoon substrate which allows the sensing structure to conform to thesurface of the test material when inserted into a pipe or other confinedspace, as illustrated in FIG. 16. Here, moving the sensing structure inan axial direction, perpendicular to the extended portions of theprimary winding, then permits complete coverage of the surface of thetest material under the sensing elements during the inspection.Measurement scans at different angular positions along the circumferenceof the hole can then provide a complete inspection of the entire holesurface.

FIG. 17 shows another embodiment for an MWM-Array, with an expanded viewof the primary meanders and the sensing elements in FIG. 18 and FIG. 19.In this case, the number of primary winding meanders 91 is reduced sothat measurements can be performed closer to material edges withoutaffecting the sensor response. The primary conductors 91 of FIG. 17 andFIG. 18 show a single wavelength for a primary winding meander. Thesecondary element arrays 72 and 76 are brought close to the centralconductors of the primary 71, so that the gap 80 between the extendedportions of the primary and secondary windings is smaller than the gap81. In this region, the magnetic field distribution is similar to thespatially periodic magnetic field distribution of a primary windinghaving more than one meander. As described in U.S. application Ser. No.09/666,879, now U.S. Pat. No. 6,657,429, and Ser. No. 09/666,524, aswell as U.S. application Ser. No. 09/891,091, filed Jun. 25, 2001, nowabandoned, the entire teachings of which are incorporated herein byreference, this structure still has the leads for the primary and thesecondary close to one another and the split primary winding design hastwo conductors in the central region 71 which also eliminates thepresence of large, extraneous external loops for linking magnetic flux.

To help reduce the series resistance for the connection leads 78 and 82the conductors are made wider in regions 93 far from the sensing regiondetermined by the extended portions of the primary winding 91. Thisreduction in series resistance reduces the ohmic heating of the primarywinding when driven by the alternating current.

Reducing the number of extended portions of the primary winding meandershas several advantages. First, since sensing elements are closer to theendmost primary winding conductors, measurements can be performed closerto the edge of a material before extended portions of the primarywinding go off the material edge and affect the measured signal. Second,the inductance of the primary winding circuit or the drive impedancealso decreases so that it is easier to drive current through theprimary, at a given voltage, at high frequencies such as 10 to 30 MHz.Third, the sensing element leads 83 cross-over a smaller number ofprimary winding conductors, which, in addition to the use of theparallel conducting loops 85, reduces the susceptibility to electricalnoise and undesired, stray magnetic flux distant from the sensingelement. The capability to measure at higher frequencies combined withthe flux cancellation lead design (83, 85, 87) permit use of smallersensing elements with low noise instrumentation, as described in U.S.application Ser. No. 10/010,062. These smaller elements (1) improvesensitivity to small defects, (2) increase the resolution for imaginginternal geometric features, such as cooling holes, corrosion orpitting, (3) reduce edge effects, (4) improve surface topology mappingcapabilities, and (5) improve coverage and quality in imaging thequality of processes such as shot peening, coating thickness andporosity, case hardening, and grinding.

Another feature illustrated in FIG. 19 is that the central portion ofthe primary winding 71 and the arrays of sensing elements 72 and 76 liein the same plane. The return legs for the primary winding 97 are on adifferent plane and connected to the central portion conductors 71 withvias at the ends of the primary winding half-meanders. This allows fordirect connections to the sensing elements with a minimum number ofvias, which improves both reliability and manufacturability at areasonable cost. Placing the critical portions of the sensor, thecentral portion of the primary winding and the secondary elements, onthe same plane also allows higher precision fabrication processes to beused. For example, standard fabrication techniques have placementtolerances between copper paths on the same layer of 3 mils (75micrometers). In contrast, the layer to layer alignment tolerance forcopper paths is normally up to 5 mils (125 micrometers). This improvesthe manufacturing reproducibility of the sensor array. Placing thecentral portion of the primary windings and the secondary elements onthe same plane also provides enhanced sensitivity to cracks and defects.One reason is that the distance between the primary and the secondaryelements is smaller than when the primary windings are in the backplane, which increases the inductive coupling between the primary andthe secondary. Another reason is that the eddy currents induced by theapplied field are larger when the primary is closer to the MUT.

In a similar fashion, the central portion of the primary winding couldalso be placed in the same plane as the secondary elements for thearrays having more than one meandering, as in FIG. 1 and FIG. 11. Thiswould provide the benefit of increased sensitivity to defects and onlyrequire via connections at the ends of the central primary windings. Theremaining extended portions of the primary windings can not be in theplane of the secondary elements because they would interfere with thelayout or pathways for the connection leads to the sensing elements.

In another embodiment, the linear rows of sensing elements can be offsetfrom one another, as shown in FIG. 20, so that scanning of the array ina direction perpendicular to the sensing elements ensures completecoverage of the MUT and no defects are missed in the gaps betweensensing elements. The drive on this array comprises two loops havingextended portions 92 and connected in series so that thecurrent in eachof the conductors 71 in the center of the drive is in the samedirection.

The effective spatial wavelength or the distance between the centralconductors 71 and the current return conductor 91 can be altered toadjust the sensitivity of a measurement for a particular inspection. Forexample, a sensor array such as FIG. 20 can be scanned over the surfaceof an MUT to inspect for surface breaking flaws or flaws hidden beneathmaterial layers. For the sensor array of FIG. 20, the distance 80between the secondary elements 72 and the central conductors 71 issmaller than the distance 81 between the sensing elements 72 and thereturn conductor 91. Modeling can be performed to calculate the responseof the flaw as it passes beneath a single element of the array, as shownin FIG. 21. This two-dimensional analysis assumed a given platethickness, a conductivity 17.4 MS/m, a lift-off of 0.15 mm, and arectangular surface breaking notch. The position of the return conductorwas also set in the model. The transimpedance between the secondary onone side of the central conductor and the drive current was calculatedfor various positions beneath the sensor array and used to determine asignal-to-noise-ratio using the formula

${S\; N\; R} = \sqrt{\left( \frac{m - m_{o}}{n_{m}} \right)^{2} + \left( \frac{p - p_{o}}{n_{p}} \right)^{2}}$where m denotes the transimpedance magnitude, p denotes thetransimpedance phase, the subscript o denotes response from the originalunflawed material distant from the flaw, and n denotes the noise in theinstrument response. This noise is determined empirically for existingsensors and assumed to be constant as the geomtry of the sensor isvaried.

The simulation results of FIG. 21 illustrate how the primary-to-primarydistance can affect the response of the sensor as it passes over a flaw.With the standard primary-to-primary distance, FIG. 21 shows a largeindication when the flaw is between the central drive winding segmentsand the sensing element. There is also a significant peak in theresponse when the flaw is nearly beneath the return leg of the primarywinding and a minor peak above the outer conductor for the secondarywinding. As the primary-to-primary separation distance is increased, theprimary peak increases slightly and the peak associated with the returnleg of the primary is reduced. This is desirable because a larger signalis obtained from the flaw and the reduction in the distant peak helps toreduce the appearance of “ghost” signals in scan images, where multipleindications are shown for a single flaw. The minor peak above the outerconductor for the secondary winding is also enhanced as theprimary-to-primary distance is increased so that more of the signal isconcentrated in the vicinity of the sensing secondary element, whichagain reduces the “ghosting” effect. An example of a modified sensordesign is shown FIG. 22. In this sensor array, all of the sensingelements 76 are on one side of the central drive windings 71. The sizeof the sensing elements and the gap distance 80 to the central drivewindings 71 are the same as in the sensor array of FIG. 20. However, thedistance 81 to the return of the drive winding has been increased, ashas the drive winding width to accommodate the additional elements inthe single row of elements.

In some applications, such as aircraft lap joint inspection for cracksor corrosion or weld inspection for stress or defects, it is desirableto map or image the properties of the MUT across the entire region ofinterest with a single scan pass and for extended distances. Rasterscanning a single element sensor across the zone of interest and downthe length of the inspection region can provide a high resolution imageof the MUT properties both across and along the inspection region, butis very time consuming. In contrast, longitudinal scanning with a lineararray of sensing elements, which provides information about the MUTproperties in the transverse direction, can be much more efficient. Thenumber, size and location of the sensing elements in the array determinethe transverse resolution of the property image created by the arrayacross the inspection region. The scan speed and data acquisition ratedetermine the resolution in the longitudinal, scan, direction. Whenthere are characteristic features of the MUT properties across theinspection region that indicate the quality of the region, the array ofsensing elements can be tailored for that particular type of inspection.

As an example, consider the inspection of a friction stir weld (FSW).The formation of an FSW is characterized by complex metal flow patternsand microstructural changes. Three distinctly different major zones canbe typically identified as: (1) a dynamically recrystallized zone (DXZ),or weld nugget, (2) a thermomechanical or heat- and deformation-affectedzone (TMZ), adjacent to the weld nugget on both leading and trailingsides of the joint, and (3) a heat-affected zone (HAZ) (Arbegast, 1998;Ditzel, 1997). The two types of defects that have been noted in frictionstir welds are: (1) tunnel defects within the nugget and (2) lack ofpenetration (LOP) (Arbegast, 1998). LOP exists when the DXZ does notreach the backside of the weld due to inadequate penetration of the pintool. The LOP zone may also contain a well-defined cracklike flaw suchas a cold lap, which is formed by distorted but not bonded originalfaying, i.e., butt, surfaces. This occurs as a result of insufficientheat, pressure and deformation. However, the LOP can be free ofwell-defined cracklike flaws, yet not be transformed by the dynamicrecrystallization mechanism since temperatures and deformation in theLOP may not be high enough. Although it may contain a tight “kissingbond,” this second type of LOP defect is the most difficult to detectwith alternative methods such as phased-array ultrasonic or liquidpenetrant inspection.

For an FSW, the quality of the weld or the joint between the basematerials can be determined from features in the measurements of theelectrical conductivity profile across the joint region, as described inmore detail in U.S. application Ser. No. 09/891,091, now abandoned, aswell as in U.S. application Ser. No. 10/046,925, filed Jan. 15, 2002,the entire contents of which are incorporated herein by reference. Forexample, planar flaws can appear as sharp reductions in the electricalconductivity and, for some alloys, the width of the peaks in theelectrical conductivity profile can provide a measure of the DXZ widthand LOP. Local reductions or dips in the electrical conductivity nearthe edges of the DXZ, as illustrated in FIG. 23, can also provideinformation about the quality of the weld. In order to inspect thesewelds, the sensor array needs to be wide enough to cover the entire weldregion. In addition, differences in the base material properties, suchas the electrical conductivity, can drastically affect the propertyprofile across the weld, so it is important to have sense elementsoutside the weld zone.

A sensor array embodiment suitable for FSW inspection is shown in FIG.24. Here, most of the sensing elements 76 are located in a single row toprovide the basic image of the material properties. A small number ofsensing elements 72 are offset from this row to create a higher imageresolution in this location, which is the location of a “dip” inelectrical conductivity near the edge of the DXZ. In addition, severalother sensing elements 96 and 98 are located a distance away from themain grouping of sensing elements in order to obtain measurements of thebase material properties of the plates being joined. Alternatively, thesize of the elements in the different regions could also be varied.Other combinations or groupings of the sensing elements are also withinthe scope of this description.

In one embodiment, the number of conductors used in the primary windingcan be reduced further so that a single rectangular drive is used. Asshown in FIG. 25, a single loop having extended portions is used for theprimary winding. A row of sensing elements 75 is placed on the outsideof one of the extended portions. This is similar to designs described inU.S. Pat. No. 5,453,689 where the effective wavelength of the dominantspatial field mode is related to the spacing between the drive windingand sensing elements. This spacing can be varied to change the depth ofsensitivity to properties and defects. Advantages of the design in FIG.25 include a narrow drive and sense structure that allows measurementsclose to material edges and non-crossing conductor pathways so that asingle layer design can be used with all of the conductors in thesensing region in the same plane. The width of the conductor 91 farthestfrom the sensing elements can be made wider in order to reduce an ohmicheating from large currents being driven through the drive winding.However, this design has half the signal of the designs in FIG. 18, FIG.20, and FIG. 22.

In another embodiment, multiple rows of sensing elements are used. FIG.26 shows a single rectangular drive winding 102 with sensing elements110 and 112 outside of the drive winding and on either side of theextended portions of the rectangular drive. The distances 114 and 116between the sense elements and the drive winding are selected, asdescribed in U.S. Pat. No. 5,453,689, to provide a prescribed effectivedepth of penetration of the magnetic field into the MUT and a prescribedsensitivity to material properties or anomalies of interest. In anembodiment, the second row of sensing elements 112 is aligned with thefirst row of sensing elements 110 so that when scanning or surfacemounted the array sensing elements detect the same crack or anomalytwice as it move across or propagates across the sensor. To facilitatemeasuring the same response from sensing elements on either side of thedrive winding to an anomaly, the distances 114 and 116 should be madeequal. The current source connection 106 to the drive winding should becentered so that the distance to each of the extended portions of therectangular drive are the same. In another embodiment, shown in FIG. 27,the spacing 114 between one set of sensing elements and the drive isdifferent than for the spacing 116 for the sensing array on the oppositeside of the drive to provide two effective depths of sensitivity. Thiscan also be accomplished with the designs in FIG. 18, FIG. 20, and FIG.22. In another embodiment, shown in FIG. 28, the sensing elements 112are offset from the sensing elements 110 parallel to the extendedportions of the rectangular drive to improve coverage for scanning andimaging of material properties or anomalies. In a preferred embodiment,this offset distance is one-half the length of the sensing element thatis parallel to the extended portions of the rectangular drive.

In each of the embodiments illustrated in FIG. 26, FIG. 27, and FIG. 28,the sensing elements can be placed either within the drive or on eitherside of the drive. These sensing elements can be placed in the samelayer as the drive winding or on different layers. For sensing elementsplaced within the drive winding rectangle, the leads to the sensingelements must either be placed in a different layer than the drivewinding conductors and separated from the drive winding conductors by alayer of insulation or the leads to the sensing elements need to passthrough the back of the sensor, out of the plane formed by the drivewindings. The use of flux cancellation leads, described earlier, is alsopreferred. An embodiment showing both rows of sensing elements close toone drive winding conductor is shown in FIG. 29. The return 104 for thedrive winding is placed on a second layer. In another embodiment, shownin FIG. 30, an active or passive electronic circuit 120 is added at theopposite end of drive winding from the current source connection 106 toeither amplify the current, reduce the self-inductance of the drivewinding, reduce capacitive effects, or minimize thermal effects. In oneembodiment, an active circuit is used to alter the resonant frequency ofthe drive circuit.

In a related embodiment, the single rectangular drive with one or moresensing elements is fabricated on a flexible substrate with a foam orother conformable or fluid support substrate. This substrate holds thesensor and allows it to be pressed against a curved or flat surfaceduring scanning to measure material properties or detect defects, asdescribed in U.S. application Ser. No. 09/946,146 filed Sep. 4, 2001,now abandoned, the entire teachings of which are incorporated herein byreference. This can be accomplished for the detection of cracks orfretting damage in engine disk slots, and the detection of cracks inbolt hole or other complex shaped MUT. The sensor can also be attachedto a rigid substrate that is flat or shaped to match the curvature of anMUT. The measurements can then be performed in a non-contact scanningmode or a permanently mounted mode.

Eddy current sensor arrays are well-suited for the inspection of largeareas for materials characterization (e.g., coating thicknessmeasurements, shot peen quality assessment, and weld inspection), thedetection of surface breaking and subsurface flaws (e.g., cracks andinclusions), and the detection of hidden corrosion. These sensor arrays,shown for example in FIG. 1, FIG. 11, FIG. 20, and FIG. 24, have one ormore linear arrays of sensing elements oriented perpendicular to thescan direction. Then, a simple scan of the array provides a measurementimage of the material properties, either in the form of the rawtransimpedance magnitude and phase or in the form of effective materialproperties if processed with measurement grids. In contrast, the use ofsingle element or conventional eddy current sensors requires scanning intwo directions, which is more time consuming than a single directionscan but can provide higher resolution images than the linear array ofdiscrete elements.

FIG. 31 and FIG. 32 provide images showing distributed microcracks,small cracks and visible macrocracks in an aluminum bending fatiguespecimen. The images are taken with the sensor in two differentorientations to demonstrate the effect of the induced eddy currentorientation on the sensitivity to cracks. For these specimens, thedistributed small cracks are dominantly oriented perpendicular to theaxis of the specimen (parallel to the bending moment axis).Consequently, FIG. 32 shows the regions of microcracking moreprominently than FIG. 31.

FIG. 33 provides the “measurement grid” used to estimate theconductivity and lift-off from the transinductance magnitude and phasedata for each sensing element of the MWM-Array. For this grid, the twounknowns are the conductivity and lift-off. In this case, the modelassumes the aluminum layer is an infinite half space. The data shown inFIG. 33 is for a single channel of the MWM-Array from the scan in FIG.31.

An example subsurface defect detection application is the inspection ofthe C-130 flight deck chine plate for hidden corrosion. The corrosiontypically occurs on the inaccessible backside of the plate while theexposed surface of the chine plate may contain, with areas of manualmaterial removal by grinding. The plate thickness between thereinforcing ribs (stiffeners) normally ranges between 0.043 and 0.047in.

An image of the plate thickness obtained from a scan with an MWM-Arrayis shown in FIG. 34. Another plate thickness image is shown in FIG. 35,with FIG. 36 showing the same data with a scale highlighting low tointermediate corrosion loss regions. A measurement grid is used toconvert the magnitude and phase measurements at each sensing elementinto estimates of plate thickness and lift-off, where lift-off is theproximity of the sensor to the outer metal surface, includingcontributions from roughness and paint. The result is a lift-offcorrected image of the plate thickness. This permits scanning withoutpaint removal, which is essential for the chine plate inspectionapplication. Note that the numbers along the vertical axis in the imagescorrespond to channel numbers. Each channel covers a 0.1-in wide area.When the MWM-Array partly overhangs the edge of the chine plate, imagingof internal geometric features and material loss close to complexfeatures such as edges and integral stiffeners is possible. Materialloss on inaccessible surface around one of the fastener holes, of 15percent to 40 percent, is readily apparent from the image. Other workhas shown that surface corrosion on the accessible surface that wasmanually ground out is also detectable; in some cases 50 percent tonearly 100 percent of the material has been removed in an attempt toremove the corroded areas. One new capability provided by the use ofabsolute sensing elements with long linear drive segments is thereduction of edge effects. By making the sensing elements small, defectsand properties near and even at edges can be imaged.

Measurements performed on simulated corrosion test specimen have alsodemonstrated the capability of the MWM-Array to quantify and locatehidden material loss. As an example, measurements were performed on atwo-layer test specimen simulating hidden corrosion in a lap joint,where the simulated material loss had a dome-shaped area machined out ofone of the layers. A plate of uniform thickness then covered the domedcutout region. Measurement scans with the MWM-Array were performed onboth sides of the plate so that the simulated material loss could be ineither the first layer, nearest the sensor, or the second layer,farthest from the sensor. Each plate had a nominal thickness of 1 mm(0.040-in) and was fabricated from an aluminum alloy.

FIG. 37 shows images of the corrosion loss in the 5 percent lossspecimens for loss in the first and second layer taken at a frequency of10 kHz. These scan images illustrate the high resolution imagingcapability of the MWM-Array and demonstrates its high sensitivity tomaterial loss of 5 percent, with apparent sensitivity to material lossbelow 1 percent and relative thickness resolution potentially to a smallfraction of a percent. Similar measurements were performed on higherloss samples, including 10 percent, 20 percent, and 30 percent loss.FIG. 38 and FIG. 39 show the responses of a single channel of theMWM-Array to material loss between two layers as the element is scannedacross the loss region. For first or second layer material loss, thenature of the MWM-Array response varies significantly with material losslocation. This variation of the response with position provides anindication of the layer in which the loss is occurring and also showsthat improper assumptions regarding the location of the corrosion lossmay result in errors in the material loss estimates. For corrosiondetection alone, this may not be important. However, erroneousassumptions will affect sensitivity and robustness, and, forprioritization based on actual material loss percentages, it is criticalto account properly for the material loss location. FIG. 40 shows acomparison of the measurements for the material loss in the first orsecond layers. There is good quantitative agreement between the twomeasurements, indicating that using an air calibration for the sensorand measurement grids based on a reasonable model for the response overthe MUT can provide a robust measurement procedure. Multiple frequenciescan also be used to estimate multiple unknowns, including paintthickness, first layer material loss, second layer material loss,conductivity of layers, gap thickness, and Alclad layer thickness. Also,the high resolution image produced by the MWM-Array permits (1)identification and estimation of stress concentrations (K factors) thatmay limit life, (2) characterization of exfoliation corrosion damage,and (3) remaining life/damage tolerance assessments.

MWM-Arrays also provide a capability to perform bi-directional magneticpermeability measurements in a scanning mode. FIG. 41 through FIG. 44provide images of the magnetic permeability for a broken tensilespecimen of 4340 low alloy steel. The MWM-Array was scanned across andalong the gage section of a specimen broken in a tensile test and thepermeability was measured at two frequencies, 158 kHz for FIG. 42 andFIG. 44 and 1 MHz for FIG. 41 and FIG. 43. In FIG. 41 and FIG. 42 theextended portions of the primary winding were oriented parallel to theloading axis. In FIG. 43 and FIG. 44 the extended portions of theprimary winding were oriented perpendicular to the loading axis. Thisillustrates the potential to map residual stress variations produced,for example by a hard landing, in parts fabricated from carbon and lowalloy steels. Notice that the permeability images at low and highfrequencies reveal stress changes with distance from the surface. A highresidual stress region near the fracture is indicated in the images ofFIG. 43 and FIG. 44. To create these images, a permeability/lift-offmeasurement grid was used, as shown in FIG. 45, assuming a knownconductivity and an infinite half-space (i.e., the steel layer isassumed to be infinitely thick). The relationship between permeabilityand stress is described in a technical paper titled “Residual andApplied Stress Estimation from Directional Magnetic PermeabilityMeasurements with MWM Sensors” submitted to the ASME Journal PressureVessels and Piping, the entire teachings of which are incorporatedherein by reference. Also, the MWM has demonstrated a capability toassess grinding process quality and detect carbide content and othermetallurgical and material features of interest. Since the lift-off ordistance between the sensing windings and the test material is beingmeasured through the measurement grids, the residual stress measurementcan be performed in a non-contact mode, which ensures that the sensorand probe assembly do not influence the stress distribution on thecomponent.

The MWM construct itself was also designed to have reduced sensitivityto its own temperature so that it could operate in elevated temperatureenvironments. The temperature affects the conductivity of the windingconductors that, in turn, affects the current distribution in theconductors. The sensitivity to winding conductivity variations withtemperature is minimized by maintaining a sufficient gap between theprimary and secondary windings. Then, the transverse diffusion ofcurrents, in which the currents in the primary winding crowd out towardsthe winding surfacers, does not cause significant increases in inductivecoupling between the primary and secondary, as described in U.S. Pat.No. 5,453,689. This also permits the use of MWM sensors and sensorarrays to measure the temperature of components. Preferably, this isdone in a non-contact mode to minimize any perturbations in the thermalenvironment; in a contact mode, thermal heat transfer through the sensorand probe assembly could significantly affect the temperature of thecomponent and any treatment being performed.

In another embodiment, the sensors can be designed using automated toolsincorporating layout rules for the conductor pathways. This tool takesinput information for the dimensions and quantity of the drive and senseelements and automatically draws the sensor layout using rules for asensor family, such as a single element MWM, an MWM-Array, or a Rosette.In one implementation, a Matlab script processes the input informationand passes it to AutoCad for the rendering the sensor design.

Another application well-suited to conformable eddy current sensorarrays is the permanent mounting of sensors in difficult-to-accesslocations. This provides an inspection capability that safely supportslife extension for aging structures and reduces weight andmaintenance/inspection costs for new structures that require both rapidand cost effective inspection capabilities. In particular, continuousmonitoring of crack initiation and growth requires the permanentmounting of sensors to the component being monitored and severely limitsthe usefulness of calibration or reference standards, especially whenplaced in difficult-to-access locations on aging or new structures.Furthermore, in many difficult-to-access locations, the actualinspection is relatively short and the costly, time-consuming part isthe disassembly to permit access to the location or surface preparationto remove, for example, sealant layers. In one embodiment, thecapability to measure stress, through permeability, is combined withpermanently mounted sensors to provide a contact or non-contact stressmeasurement capability.

Conventional eddy current designs are not ideal for permanent mounting.Conventional eddy-current techniques require varying the proximity ofthe sensor (or lift-off) to the test material or reference part byrocking the sensor back and forth or scanning across a surface toconfigure the equipment settings and display. For example, for crackdetection the lift-off variations is generally displayed as a horizontalline, running from right to left, so that cracks or other materialproperty variations appear on the vertical axis. Affixing or mountingthe sensors against a test surface precludes this calibration routine.The probe-to-probe variability of conventional eddy-current sensorsprevents calibrating with one sensor and then reconnecting theinstrumentation to a second (e.g., mounted) sensor for the test materialmeasurements. These shortcomings are overcome with spatially periodicfield eddy-current sensors that provide absolute property measurementsand are reproduced reliably using micro-fabrication techniques.Calibrations can also be performed with duplicate spatially periodicfield sensors using the response in air or on reference parts prior tomaking the connection with the surface mounted sensor. The capability tocharacterize fatigue damage in structural materials, along with thecontinuous monitoring of crack initiation and growth, has beendemonstrated, as described in U.S. application Ser. No. 09/666,879, nowU.S. Pat. No. 6,657,429, and Ser. No. 09/666,524. This inspectioncapability is suitable for on-line fatigue tests for coupons and complexcomponents, as well as for monitoring of difficult-to-access locationson both military and commercial aircraft.

The surface mountable MWM-Rosette shown in FIG. 46 is just one exampleof a sensor design suitable for surface mounting on aircraft. The designof surface mountable MWM-Arrays includes three requirements: (1) thesensing footprint must be large enough to cover the region of interestwithin which cracks might initiate and propagate, (2) the resolution ofthe sensing elements must be sufficient to monitor growth rates andestimate crack length (if more than just detection is required,subelement crack length variations can be estimated from the signal sizeas well), (3) at least one sensing element should be located in a regionnot likely to contain cracks during the inspection period. Three basicconstructs for surface mounted sensors may be used: (1) the MWM-Rosetteis designed for detection of cracks at fasteners as shown in FIG. 46,(2) the linear MWM-Array format shown in FIG. 47 can be used to monitorcrack initiation and growth along a linear feature, e.g., a radius in anaircraft structural, and (3) the linear array format shown in FIG. 48can be used to detect cracks that propagate across a specific locationwithin a structure member. Each of these designs can be located on anexposed surface or sandwiched between layers (e.g., skins).

FIG. 49 provides data from a fatigue test with an MWM-Rosette mountedaround a hole in an aluminum dogbone specimen. Each channel numbercorresponds to an individual annular sensing element, with channel 1being closest to the fastener and channel 7 the furthest from thefastener. FIG. 50 shows a crack growth curve based on the data shown inFIG. 49 and known MWM-Array geometry. The conductivity drop in eachchannel occurs when the crack approaches the primary winding on theinner side of the sense winding.

Other types of sensing elements can also be used in these arrays. Thesmall rectangular sensing elements 72 shown, for example in FIG. 2,could be super-conducting SQUID type sensors, Hall effect probes,magnetoresistive (MR) sensors, giant magnetoresitive (GMR) sensors, orwound eddy current sensor type coils. A representative sensor that usesa GMR sensor as a sensing element and a rotationally symmetricdistributed drive winding is shown in FIG. 51 and described in detail inU.S. application Ser. No. 10/045,650, filed Nov. 8, 2001, the entireteachings of which are incorporated herein by reference. For this drivewinding, the number of turns in each circular winding segment 30 isvaried to shape the field. Interconnections between each segment aremade with tightly wound conductor pairs 32 to minimize fringing fieldeffects. A GMR sensor 34, with feedback controlled coil, is placed atthe center of the concentric circular drive windings. Connections tothis hybrid sensing element are made with a tightly wound conductor pair36. Both the number of turns and the polarity of the windings (currentdirection) can be varied in the drive winding segments. In this case,there are two sets of drive windings which allows more than onefundamental spatial mode. The polarity of the connection determineswhich of the two current drive patterns (with different fundamentalspatial wavelengths) is excited. This provides two distinct field depthsof penetration conditions and permits improved multiple propertymeasurements for layered media.

Once the sensor response is obtained, an efficient method for convertingthe response of the GMR sensor into material or geometric properties isto use grid measurement methods. These methods map the magnitude andphase of the sensor response into the properties to be determined. Thesensors are modeled, and the models are used to generate databasescorrelating sensor response to material properties. The measurementgrids are two-dimensional databases that can be visualized as “grids”that relate two measured parameters to two unknowns, such as theconductivity and lift-off (where lift-off is defined as the proximity ofthe test material to the plane of the sensor windings). For coatingcharacterization or for inhomogeneous layered constructs,three-dimensional grids (or higher order grids), called lattices (orhyper-cubes), are used. Similarly, a model for the GMR sensor withfeedback loop and circular drive windings was developed and used togenerate measurement grids, which were then used to interpret sensorresponse. Alternatively, the surface layer parameters can be determinedfrom numerical algorithms that minimize the least-squares error betweenthe measurements and the predicted responses from the sensor.

An advantage of the measurement grid method is that it allows forreal-time measurements of the absolute electrical properties of thematerial. The database of the sensor responses can be generated prior tothe data acquisition on the part itself, so that only table lookupoperation, which is relatively fast, needs to be performed. Furthermore,grids can be generated for the individual elements in an array so thateach individual element can be lift-off compensated (or compensated forvariation of another unknown, such as permeability or coating thickness)to provide absolute property measurements, such as the electricalconductivity. This again reduces the need for extensive calibrationstandards. In contrast, conventional eddy-current methods that useempirical correlation tables that relate the amplitude and phase of alift-off compensated signal to parameters or properties of interest,such as crack size or hardness, require extensive calibrations andinstrument preparation.

Several sets of measurements have been performed with a circularlysymmetric shaped field magnetometer. These measurements used the GMReddy current sensor with drive illustrated in FIG. 51. A simpleone-point air calibration method is used for all of these measurements.This means that the sensor response when over the test material wasnormalized by the sensor response in air, away from any conducting ormagnetic materials. The measurement results are then processed withmeasurement grids to provide absolute property measurements, such aselectrical conductivity, magnetic permeability, material thickness, andsensor proximity (lift-off). The absolute property measurementcapability eliminates the need for extensive, and in some cases any,calibration sets. Even if reference calibrations are performed, possiblyto improve the accuracy of the property estimation, only a singlecalibration material may be required. Air and reference part calibrationmethods have previously been described for square wave meanderingwinding constructs in U.S. Pat. No. 6,188,218, the contents of which arehereby incorporated in its entirety. The discrete segment Cartesian andcircular geometry sensors described herein can also be calibrated inthis fashion because the sensor response can be accurately modeled. Inprinciple, air calibrations in this context can be performed with anysensor whose response can be accurately modeled.

FIG. 52 shows the measurement grid for conductivity/lift-offmeasurements with three different materials, in the form of metalplates, over a range of lift-off values. Since both the conductivity andthe lift-off parameters vary over a relatively large range, theparameter values for this grid are chosen on a logarithmic scale. Thegrid cell area is a measure of the sensitivity of the measurement inthat region of the grid. The measurements are carried out at 12.6 kHz.Placing plastic shims between the sensor and the metal plates varied thelift-off. The three data sets follow lines of constant conductivity veryclosely. As listed in Table 1, the measured lift-off values were inexcellent agreement with the nominal values. Only the first 12 sets arelisted, due to the lack of sensitivity at higher lift-off values, asillustrated by the narrowing of the grid cells in FIG. 52.

The lowest value of the lift-off, 3.3 mm, corresponds to measurementswith no shim, and is equal to the effective depth of the windings belowthe surface of the sensor. This amount has been added to the data in thelast column, after having been estimated by taking the average of thedifference between the magnetometer estimated values and the measuredshim thicknesses. This number is quite reasonable, given that theaverage depth of the grooves is on the order of 3 mm, and that thewinding thickness, about 2 mm, is not considered by the model. Theconductivity data in Table 1 are also in good agreement with valuesreported in the literature. There appears to be an optimal range of thelift-off, 5–7 mm, where the estimated conductivity is most accurate.This is reasonable since sensitivity is lost at higher lift-offs, whilea close proximity to the sensor windings is also not desirable since theeffects of the non-zero winding thickness then become more significant.These conductivity results are also remarkable good considering thatthis measurement was carried out with no calibration standards and witha single air calibration point, the model for the sensor response isrelatively simple, and no empirical data have been used to determine thesensor response. If it is necessary to perform a very exact conductivitymeasurement, then a two-point reference part calibration is recommended,with the properties of the two reference parts (or the same part at twolift-off values) bracketing the properties of the unknown part. Theseresults confirm the validity of the model for this cylindricalcoordinate sensor.

TABLE 1 Measurement results corresponding to FIG. 52. Conductivity[MS/m] Lift-off [mm] Nominal Data Cu Cu Lift- Set 110 Al 6061 Al 2024110 Al 6061 Al 2024 off [mm]  1 59.2 29.5 18.0 3.2 3.3 3.3 3.3  2 59.228.9 17.8 4.0 4.1 4.1 4.1  3 58.7 28.7 17.8 4.7 4.8 4.5 4.8  4 58.3 28.617.6 5.5 5.6 5.6 5.6  5 57.8 28.3 17.6 6.4 6.5 6.5 6.5  6 57.1 28.1 17.57.3 7.1 7.3 7.3  7 55.7 27.4 17.3 7.9 8.0 8.0 8.0  8 56.1 27.5 17.4 8.78.9 8.8 8.8  9 54.3 26.8 17.1 9.4 9.5 9.4 9.4 10 55.2 27.0 17.2 10.210.3 10.3 10.2 11 53.5 26.4 17.0 10.8 10.9 10.9 10.9 12 53.0 26.3 16.711.7 11.7 11.7 11.7

Another set of measurements illustrates the GMR magnetometer capabilityto detect material flaws in a thick layer of metal. These measurementswere carried out by performing scans over a set of stainless steelplates. One plate had a 25 mm long, 0.4 mm wide, and 2.4 mm depth slotto simulate a crack. The crack is not modeled explicitly, but itspresence is usually manifested by a local reduction in the value of themeasured conductivity. In some cases, depending on its depth andposition below the surface, it may appear as a local change in thelift-off. Several sets of scans were made with stainless steel platesarranged to simulate a crack at the upper surface, nearest the sensor, acrack 3.2 mm below the upper surface, and a crack 7.2 mm below thesurface. The image generated by one scan, with the slot at the surface,is shown in FIG. 53. This image shows the conductivity, normalized byits value away from the crack. The crack signal is very strong, with theconductivity decreasing more than 3% near the crack position. The doublehump signature of the crack is characteristic of the effect cracks haveon the signal of imposed-periodicity eddy current sensors. The inducedcurrent density mirrors the current density of the drive, and as aconsequence, the disruption caused by the crack is greatest when it isdirectly below, and perpendicular, to the primary winding nearest to thesensing element. For deeper cracks, near the crack, the measuredconductivity is actually higher. This is because the phase of theinduced eddy currents changes with depth. With the crack positioned 7.2mm below the surface it interrupts eddy currents that are flowing in adirection opposite to the surface eddy currents, thereby increasing themagnetic field at the sensor. A consequence of this effect is that thereis a characteristic depth, near π/2 skin depths, where a crack wouldcause no change in the conductivity.

A GMR sensor can be placed in a feedback configuration with a secondarywinding, as shown in FIG. 54. In this way the magnetic field at the GMRsensor remains nearly constant during operation, eliminating the effectof the nonlinear transfer characteristic, while maintaining sensitivityat low frequencies. The magnitude of the current in the secondarywinding is taken as the output signal, and since the relationshipbetween this current and the magnetic field for an air-core winding islinear, so is the transfer characteristic of the entire hybrid sensorstructure. The magnetic field magnitude that this hybrid GMR sensor canmeasure is limited only by the magnitude of the field that the secondarywinding can produce, which can be orders of magnitude higher than thesaturation field of the GMR sensor. This dramatically increases thedynamic range of the GMR sensor and makes its use far more practicalthan in alternative implementations with permanent magnets orelectromagnets that provide a constant bias.

Another benefit of the feedback configuration is temperature stability.Since the measured quantities are currents in the windings, which aredirectly related to the magnetic fields, temperature dependence of theGMR sensor on winding resistance, etc. has no effect on the magnetometerresponse. This is critical since temperature variations have limitedreproducibility and limit the use of many commercially available eddycurrent arrays. Goldfine and Melcher (U.S. Pat. No. 5,453,689) solvedthe temperature sensitivity problem for inductive sensing elements bymaintaining a gap between drive and sensing windings. Temperaturestability is a key to the practical use of GMR sensors as well.

Another advantage of the feedback connection is for biasing the GMRsensor. Biasing the GMR sensor to the appropriate operating point isaccomplished simply by adding an appropriate DC voltage offset at theinput of the gain stage. This is much better than the alternativebiasing methods described earlier, since correct biasing is maintainedeven if the position of the GMR sensor with respect to the bias sourcechanges, which would not be true for biasing with a constant fieldsource. This eliminates the need for complex alignment methods, sincebiasing at the correct level is automatic with the appropriate choice ofcircuit components. As a result, this feedback configuration providesthe same sensitivity of a GMR sensor by itself while maintaining alinear transfer characteristic and a wider dynamic range.

The position of the GMR elements within the feedback coil, and theposition of the feedback coil within the primary winding can also beadjusted. FIG. 55 illustrates that one or more GMR sensors 84 can besurrounded by a feedback coil 82 and placed at the center of a drivewinding 80. The use of multiple GMR sensors within the footprint of thedrive winding promotes imaging of material properties when the array isscanned in a direction perpendicular to the row of GMR sensors. The useof a single feedback coil and multiple GMR sensor elements eliminatescross-talk between elements, which may occur if each GMR element has itsown feedback coil, and also simplifies the drive circuitry for thesensor array. FIG. 56. shows a similar array with the row of GMRelements 84 and feedback coil offset so that it is closer one side ofthe primary winding than the other. This results in an asymmetricresponse when the array is scanned over a flaw since the array is moresensitive to the effects of the flaw when it passes beneath the nearerportion of the primary winding. Similarly, sensing elements can beplaced outside of the drive winding, as illustrated in FIG. 57, wherethe row of sensor elements 84 is far from the drive winding 80 while asecond row of sensors 86 is near the drive winding. An advantage of thisconfiguration is that any connection leads to the sensing elements doesnot have to pass over the conductors of the drive winding, which helpsto minimize parasitic responses.

The inventions described here relate to methods and apparatus for thenondestructive measurements of materials using sensors that applyelectromagnetic fields to a test material and detect changes in theelectromagnetic fields due to the proximity and properties of the testmaterial. Although the discussion focused on magnetoquasistatic sensors,many of the concepts extend directly to electroquasistatic sensors aswell.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

The following references are incorporated herein by reference in theirentirety:

-   Arbegast, W. J., and Hartley, P. J. (1998), “Friction Stir Weld    Technology Development at Lockheed Martin Michoud Space, Systems—An    Overview”, 5^(th) International EWI Conference on Trends in Welding    Research, 1–5 Jun., 1998, Pine Mountain, Ga.-   Auld, B. A. and Moulder, J. C. (1999), “Review of Advances in    Quantitative Eddy-Current Nondestructive Evaluation,” Journal of    Nondestructive Evaluation, vol. 18, No. 1.-   Ditzel, P., and Lippold, J. C. (1997), “Microstructure Evolution    During Friction Stir Welding of Aluminum Alloy 6061-T6”, Edison    Welding Institute, Summary Report SR9709.    The following references are also incorporated herein by reference    in their entirety:-   1. Navy Phase I Proposal, titled “Wireless Communications with    Electromagnetic Sensor Networks for Nondestructive Evaluation”,    Topic #N01-174, dated Aug. 13, 2001.-   2. Air Force Phase I Proposal, titled “Three-Dimensional Magnetic    Imaging of Damage in Multiple Layer Aircraft Structures”, Topic    #AF02-281, dated Jan. 14, 2002.-   3. Final Report submitted to FAA, titled “Crack Detection Capability    Comparison of JENTEK MWM-Array and GE Eddy Current Sensors on    Titanium ENSIP Plates”, dated Sep. 28, 2001, Contract    #DTFA03-00-C-00026, option 2 CLIN006 and 006a.-   4. Technical Paper titled “Surface Mounted and Scanning Periodic    Field Eddy-Current Sensors for Structural Health Monitoring”,    presented at the IEEE Aerospace Conference, March 2002.-   5. Technical Paper titled “Corrosion Detection and Prioritization    Using Scanning and Permanently Mounted MWM Eddy-Current Arrays”,    presented at the Tri-Service Corrosion Conference, January 2002-   6. Technical Paper titled “Shaped-Field Eddy Current Sensors and    Arrays”, presented at the SPIE Conference, March 2002.-   7. Technical paper titled “Residual and Applied Stress Estimation    from Directional Magnetic Permeability Measurements with MWM    Sensors,” submitted to ASME Journal Pressure Vessels and Piping.-   8. Technical paper titled “MWM-Array Characterization and Imaging of    Combustion Turbine Components,” EPRI International Conference on    Advances in Life Assessment and Optimization of Fossil Power Plants,    Orlando, Fla.; March 2002.-   9. Presentation slides “Fatigue Test Monitoring and On-Aircraft    Fatigue Monitoring Using Permanently Mounted Eddy Current Sensor    Arrays,” USAF ASIP Conference, Williamsburg, Va., December 2001.-   10. Technical presentation slides “Condition Assessment of Engine    Component Materials Using MWM-Eddy Current Sensors,” ASNT Fall    Conference, Columbus, Ohio; October 2001.-   11. Technical presentation slides “High-Resolution Eddy Current    Sensor Arrays with Inductive and Magnetoresistive Sensing Elements,”    ASNT Fall Conference, Columbus, Ohio; October 2001.-   12. Technical presentation slides “Surface Mounted MWM-Eddy Current    Sensors for Structural Health Monitoring,” ASNT Fall Conference,    Columbus, Ohio; October 2001.-   13. Technical paper and presentation slides titled “High Throughput,    Conformable Eddy-Current Sensor Arrays for Engine Disk Inspection    including Detection of Cracks at Edges and in Regions with Fretting    Damage,” NASA/FAA/DoD Conference on Aging Aircraft, Kissimmee, Fla.;    September 2001-   14. Technical paper and presentation slides titled “High-Resolution    Eddy Current Sensor Arrays for Detection of Hidden Damage including    Corrosion and Fatigue Cracks,” NASA/FAA/DoD Conference on Aging    Aircraft, Kissimmee, Fla.; September 2001.-   15. Technical paper titled “Flexible Eddy Current Sensors and    Scanning Arrays for Inspection of Steel and Alloy Components,”    7^(th) EPRI Steam Turbine/Generator Workshop and Vendor Exposition,    Baltimore, Md.; August 2001.-   16. Technical paper titled “Conformable Eddy-Current Sensors and    Arrays for Fleet-wide Gas Turbine Component Quality Assessment,”    ASME Turbo Expo Land, Sea & Air, New Orleans, La.; June 2001.-   17. Technical presentation slides titled “Friction Stir Weld LOP    Defect Detection Using New High-Resolution MWM-Arrays and MWM    Eddy-Current Sensors,” Aeromat 2001 Conference; June 2001.-   18. Technical paper titled “Applications for Conformable Eddy    Current Sensors including High Resolution and Deep Penetration    Sensor Arrays in Manufacturing and Power Generation,” ASME 7^(th)    NDE Topical Conference, San Antonio, Tex.; 2001.-   19. Technical paper titled “Surface Mounted Periodic Field Current    Sensors for Structural Health Monitoring,” SPIE Conference: Smart    Structures and Materials NDE for Health Monitoring and Diagnostics,    Newport Beach, Calif.; March 2001.-   20. Technical paper and presentation “Scanning and Permanently    Mounted Conformable MWM Eddy Current Arrays for Fatigue/Corrosion    Imaging and Fatigue Monitoring,” USAF ASIP Conference, San Antonio,    Tex., December 2000.-   21. Technical presentation slides “Inspection of Gas Turbine    Components Using Conformable MWM Eddy-Current Sensors,” ASNT Fall    Conference, Indianapolis, Ind.; November 2000.-   22. Technical paper titled “Anisotropic Conductivity Measurements    for Quality Control of C-130/P-3 Propeller Blades Using MWM Sensors    with Grid Methods,” Fourth DoD/FAA/NASA Conference on Aging    Aircraft, St. Louis, Mo.; May 2000.-   23. Technical paper titled “Surface-Mounted Eddy-Current Sensors for    On-Line Monitoring of Fatigue tests and for Aircraft Health    Monitoring,” Second DoD/FAA/NASA Conference on Aging Aircraft,    August 1998.-   24. Technical paper titled “Early Stage Fatigue Detection with    Application to Widespread Fatigue Damage Assessment in Military and    Commercial Aircraft,” First DoD/FAA/NASA Conference on Aging    Aircraft, Ogden, Utah, June 1997.-   25. Technical paper “Combustion Turbine Blade Coating    Characterization Using a Meandering Winding Magnetometer,” ASNT Fall    Conference, 1994.

1. A test circuit comprising: a primary winding of parallel conductingsegments having extended portions including at least one centralconductor and at least one return conductor positioned on either side ofthe central conductor to impose a magnetic field in a test material whendriven by an electric current; a plurality of sense elements for sensingthe response of the test material to the imposed magnetic field, eachsensing element positioned between the extended portions of the primarywinding, the sense elements being aligned with one another to sense theresponse at incremental areas along a path parallel to the extendedportions of the primary winding, and having separate output connections.2. A test circuit as claimed in claim 1 wherein the distance between thecentral conductors and return conductors are selected to align withfeatures of a component being tested.
 3. A test circuit as claimed inclaim 1 including two central conductors and two return pathssymmetrically located on either side of the central conductors.
 4. Atest circuit as claimed in claim 3 wherein the distance between thecentral conductors and return conductors are selected to align withfeatures of a component being tested.
 5. A test circuit as claimed inclaim 1 further comprising a second plurality of sense elements alignedwith one another to sense the response at incremental areas along a pathparallel to the extended portions of the primary winding, and havingseparate output connections.
 6. A test circuit as claimed in claim 5wherein each individual sense element in the first plurality of senseelements is aligned with a sense element in the second plurality ofsense elements in a direction perpendicular to the extended portions ofthe primary winding.
 7. A test circuit as claimed in claim 5 wherein thesense elements in the first plurality of sense elements is offset in adirection parallel to the extended portions of the primary winding fromthe sense elements in the second plurality of sense elements.
 8. A testcircuit as claimed in claim 7 wherein the offset distance is one-half ofthe length of a sensing element.
 9. A test circuit as claimed in claim 5wherein the distances from the first plurality of sense elements and thesecond plurality of sense elements to the central conductor are equal.10. A test circuit as claimed in claim 9 wherein a differentialmeasurement is taken between a sense element in the first plurality ofsense elements and a sense element in the second plurality of senseelements.
 11. A test circuit as claimed in claim 1 wherein the sensingelements and the central conductor are in the same plane.
 12. A testcircuit as claimed in claim 1 wherein the location of the sense elementsis non-uniform in the direction parallel to the extended portions of theprimary winding.
 13. A test circuit as claimed in claim 1 wherein theprimary winding and sense elements are fabricated onto a flexiblesubstrate.
 14. A test circuit as claimed in claim 1 further comprising aballoon filled with a fluid to maintain contact between the test circuitand a surface under test of the test material.
 15. A test circuit asclaimed in claim 14 wherein the balloon is attached to a shuttle and theshuttle is shaped to approximately match the shape of the material undertest.
 16. A test circuit as claimed in claim 15 further comprising aremovable cartridge that permits rapid replacement of the sense elementsand balloon components.
 17. A test circuit as claimed in claim 14wherein the surface under test is inside a bolt hole.
 18. A test circuitas claimed in claim 14 wherein the surface under test is inside anengine disk slot.
 19. A test circuit as claimed in claim 1 wherein theconductivity and proximity of the sensor to the surface are measured todetect cracks.
 20. A test circuit as claimed in claim 1 wherein theproximity is measured at each sensing element to determine surfaceroughness.
 21. A test circuit as claimed in claim 1 wherein each sensingelement response is used for monitoring a condition of the testmaterial.
 22. A test circuit as claimed in claim 1 wherein the primarywinding and sense elements are fabricated onto a rigid substrate.
 23. Atest circuit as claimed in claim 1 wherein the sensor is not in contactwith a surface under test of the test material.
 24. A test circuit asclaimed in claim 1 wherein at least one of the sense elements includes amagnetoresistive sensor.
 25. A test circuit as claimed in claim 1wherein the at least one of the sense elements includes a giantmagnetoresistive sensor.
 26. A test circuit as claimed in claim 25further comprising a secondary coil that surrounds the giantmagnetoresistive sensing element.
 27. A test circuit as claimed in claim26 wherein the secondary coil is in a feedback configuration.
 28. A testcircuit as described in claim 1 wherein the sensor response to a flaw isdetermined in advance, this response being used to construct a filter,and the filter being applied to a sensor response to search forindications likely to be the flaw of interest and to suppress responsesunlikely to be that flaw.